You didn't know how unpopular Jason was when you invited him to your party.
If I'd/I had known how unpopular Jason was, I wouldn't have invited him (to my party).
I haven't got a key, so I can't get in.
If I had a key, I could get in.
Warn your friend not to put too many tins into the plastic bag or it'll break.
If you put too many tins into the plastic bag, it'll break.
He's very ill so he isn't at work.
Unless he was very ill, he would be at work.
The baby didn't cry, so we had a good sleep.
If the baby had cried, we wouldn't have had a good sleep.
Nigel hasn't got satellite TV, so he can't watch the game.
If Nigel had satellite TV, he could watch/would be able to watch the game.
You may want a chat, so just give me a ring.
If you want a chat, just give me a ring.
You haven't got a pen, so you can't write down the address.
If I had a pen, I could write down the address.
You should have started your project earlier. You're so far behind now.
If I'd/I had started my project earlier, I wouldn't be so far behind (now).
You talk about football and I'm going to be very bored.
If you talk about football, I'm going to be very bored.
Throw a stone into water and it sinks.
If you throw a stone into water, it sinks, OR it'll/it will sink.
The automatic result of the door opening is the fan coming on.
If the door opens, the fan comes on.
We should have bought that vase five years ago because it would be quite valuable now.
If we'd/we had bought that vase five years ago, it would be quite valuable now.
You think Emma should book a seat on the train. Otherwise, she has to stand.
If Emma doesn't book a seat on the train, she'll have to stand.
I'm afraid of spider or I'd have picked it up.
If I wasn't afraid of spiders, I would have picked it up.
You go away and I'll miss you.
If you go away, I'll miss you.
I'm not rich or I'd have given up working long ago.
If I was/were rich, I'd/ would have given up working long ago.
We're lost because we didn't look at the map.
We wouldn't be lost if we had looked at the map.
Your friend might need some help. If so, tell her to give you a ring.
If you need some/any help, give me a ring.
It won't rain tomorrow and I'll go to the coast.
We’ll go to the coast tomorrow unless it rains.